It is known to coat steel wires with brass to promote adhesion to rubber. To improve and for the purpose of maintaining this adhesion under severe operating conditions, i.e. e.g. in humid conditions and/or at elevated temperature it has already been proposed to deposit a second metal coating, e.g. cobalt, on this brass coating as first metal coating. This measure is known from British Pat. application No. 2 076 320 A. It is also mentioned in this patent application that a cobalt coating has among other things a limited bonding affinity for brass. In order to attain a good bond between the two metal coatings it is then proposed to draw the coated object so that the cobalt diffuses into the brass surface. Apart from the fact that this method for applying the second coating still requires an additional drawing operation, cobalt is relatively hard to deform by drawing. Also, there is a risk of excessive diffusion of zinc from the brass into the cobalt, which causes the adhesion behaviour to decrease.
There is, however, an increasing need to provide the reinforcing elements with coatings with specific predetermined compositions according to i.e. the required durable adhesion with respect to special rubbers, e.g. rubbers with low sulphur content, fast vulcanising rubbers, etc. These elastomer compositions are constantly evolving so that there has arisen a need for flexibility for the bond between steel substrate and rubber via specific and relatively easy to apply intermediate layer systems. More in particular, there is a need for flexibility for adhesion layer and corrosion protection layer systems for the reinforcing substrates. A coating composition of two metal layers offers ample possibilities in this respect provided a good adhesion can be realised between first and second metal coating without adversely affecting the specific chosen layer compositions.
So, the object of the invention is, among other things, to provide steel substrates for the reinforcement of elastomers and provided with a first and a second metal coating, the intended and intrinsically present properties of first and second coating at least being maintained and at the same time a good and durable bond being guaranteed between the two coatings.
This objective is now met in accordance with the invention by providing a bonding layer between first and second metal coating that effects a durable adhesion between first and second coating, this second coating at least covering part of the first coating. As opposed to the cobalt/brass diffusion interface layer formed in accordance with British Pat. application No. 2 076 320 A the said bonding layer in accordance with the invention will comprise at least one nonmetallic component that, among other things, substantially contributes to the bond between the two metal coatings. In particular, this bonding layer can refrain the diffusion of the metals of the one coating into the adjacent coating when applying the second coating or during bonding to elastomers through vulcanisation. Also, this measure allows to dispense with the relatively difficult deformation operation afterwards, e.g. when cobalt is applied as second coating as in Pat. No. GB 2 076 320 A.
The first metal coating may consist of copper, zinc, nickel, tin, iron, chromium, manganese or alloys thereof, or else alloys thereof with cobalt, molybdenum, vanadium, titanium or zirconium. It may be intended to promote the adhesion of the steel substrate to vulcanisable elastomers. It may, possibly at the same time, be intended as corrosion-protection layer or frictional-resistance reducing coating or wear-resistant layer. A very common adhesion promoting coating is brass with in total between 58%-70% by weight of copper and the remainder being zinc. However, the copper content at the brass-layer surface will preferably be much lower, e.g. corresponding to a (Cu/Cu+Zn)-ratio not exceeding 20% when Cu and Zn are expressed in at.%.
The second metal coating may comprise one or more of the elements nickel, tin, iron, chromium, manganese, molybdenum or cobalt. It may among other things be intended to enhance the adhesion of the steel substrate, in particular the durability of the adhesion in critical operating conditions, as for cobalt. It may fulfil other functions as well, such as e.g. those enumerated hereinbefore in connection with the first coating.
A nonmetallic component of the bonding layer may be oxygen but also phosphor or nitrogen. Oxygen will be a preferred component for certain specified first and second coating types. Here, the bound oxygen may occur as oxide. In particular, it may occur as metal oxide in the bonding layer e.g. as oxide of a metal of the first coating. When e.g. brass is applied first metal coating a bonding layer mainly consisting of zinc oxide will effect a good bonding affinity for a secondcoating of cobalt, particularly when the (Cu/Cu+Zn)-ratio at the brass surface is relatively low. Preferably, the absolute copper content at the surface of the bonding layer will be less than 25 at.%. In other cases, titaniumnitride may be suitable as bonding layer. Obviously, the bonding layer may also fulfil other functions in addition to its inherent bonding function, e.g. increasing corrosion resistance, resistance to corrosion fatigue, ductility, wear resistance, etc.
A method for manufacturing the steel substrate coated in accordance with the invention basically comprises the application onto it of a first metal coating, followed by the application or the formation of a bonding layer on at least part of the first coating surface whereupon a second metal coating is applied onto the bonding layer Before applying or forming the bonding layer the substrate with the first coating may be subjected to a heat treatment and/or mechanical deformation operation to obtain a desired composition, thickness and structure However, after applying the second coating a thermomechanical consolidation treatment of the coatings with the intermediate bonding layer is no longer required, as opposed to what is known from British Pat. application No. 2 076 320 A.
Preferably, the method will be carried out in a continuous process. In particular, it has been found convenient to conduct a substrate, which has already been provided with the first metal coating with the desired composition, thickness and structure, through a coating installation in an continous process and to coat it there with the desired bonding layer and second metal coating. It will thereby be possible for the second coating to cover the bonding layer wholly or partly.
This continuous process can also be carried out in line for e.g. one or more parallel running substrates with a directlysucceeding bonding operation to a vulcanisable elastomer, e.g. by calendering and for the continuous reinforcement of the elastomer sheet. The feed-through speed in the coating installations must then of course be adapted to that of the calendering.
In particular, an oxidic bonding layer in accordance with the invention can be formed by suitably oxidising the substrate, provided with a first metal coating, in a way known in itself, either thermally to the air, or physically in a plasma or e.g. chemically. The bonding layer does not necessarily have to cover the total surface of the substrate, but basically only that part that has to be coated with the second coating afterwards. Sometimes it may even be appropriate to oxidise only that particular part when an oxide layer on the rest of the surface of the first coating would adversely affect the required properties of this surface, e.g. cause its adhesive capacity to decrease.
Basically, the second metal coating only needs to cover the part of the steel substrate that has to be very firmly and durably bonded to the elastomer afterwards. In the case of a steel cord, for instance, it is extremely important that the visible outside surface of the cord (i.e. of the outside or mantle filaments) have a good adhesive capacity. The adhesion of the elastomer to the core filaments is often less critical. Consequently, a common brass will usually suffice for the first metal coating.